Cyclohexene, 1,6-dibromo

Cyclohexanone, 5-methyl-2-(2-propenyl)-[36300-104], 55 Cyclohexene [110-83-8], 34 Cyclohexene, 1,6-dibromo- [17202-32-3],... 

Cyclohexanones, 2-alkyl-5 methyl-, 56 Cyclohexene, 34 Cyclohexene, 1,6-dibromo-, 34 CYCLOHEXENE, 3-METHYL-, 101 Cyclohexene, 1-phenyl- [Benzene, (1-eyclohexen-l-yl)-], 106 2-Cyclohexen-l-ol, 2-bromo-, 34 2-Cyclohexen-l-ol, 3-methyl-, 101 2-Cyclohexen-l-one, 2-allyl-3-methyl-[2-Cyclohexen-l-one, 3-methyl-2-(2-piopenyl)-], 55... 

Although benzene is clearly unsaturatcd, it is much more stable than typical alkenes and fails to undergo the usual alkene reactions. Cyclohexene, for instance, reacts rapidly with Br2 and gives the addition product 1,2-dibromo-cyclohexane, but benzene reacts only slowly with Br2 and gives the substitution product CgH Br. As a result of this substitution, the cyclic conjugation of the benzene ring is retained. 

Cyclohexadiene has been prepared by dehydration of cyclohexen-3-ol,3 by pyrolysis at 540° of the diacetate of cyclohexane-1,2-diol,4 by dehydrobromination with quinoline of 3-hromocyclohexene,6 by treating the ethyl ether of cyclohexen-3-ol with potassium bisulfatc,6 7 by heating cyclohexene oxide with phthalic anhydride,8 by treating cyclohexane-1,2-diol with concentrated sulfuric acid,9 by treatment of 1,2-dibromocyclo-hexane with tributylamine,10 with sodium hydroxide in ethylene glycol,10 and with quinoline,6 and by treatment of 3,6-dibromo-cyclohexene with sodium.6... 

Similarly, cyclohexene will form 1,2-dibromo-cyclohexane as a racemic product, again R,R and S,S. Note that the three-membered ring of the bromonium ion must be planar and can only be defused to the cyclohexane ring (see Section 3.5.2). 

Recently, the successful generation of PCU-8-vinylidenecarbene (4a) via reaction of 8-(dibromo-methylene)-PCU (3) with n-BuLi hs been reported [15]. When this reaction is performed in the presence of an alkene trapping agent (i.e., cyclohexene), a cage-functionalized erro-methylenecyclopropane, 5, is the only product. Compound 5 subsequently was characterized via conversion to the corresponding substituted dichlorospiro(cyclopentane), 6 (Scheme 2) the structure was established unequivocally via single-crystal X-ray structural analysis [15]. 

The chemical shifts (downfield from 85% HoPO ) of sulfonium salts ]X with various Y are very close (a) 5 = +86.1 ppm (temp. 55°C), (b) 6 = +90.1 ppm, (c) 6 = +86.8 ppm, (d) 6 = +86.7 ppm, (e) 6 = +89.2 ppm, (f) 6 = +92.2 ppm. This suggests that all these salts have "true" sulfonium structure with little interaction within the ion pair involved. It is of interest to note that J X (X = Y = Br) reacts with cyclohexene to give 1,2-dibromo-cyclohexane and the starting thiolate. Sulfonium salts J X react readily with external nucleophiles of high P-nucleophi1icity e.g. water and alcohols. The reaction between l (R = Bu1, R = Ph,... 

That 1,2-cyclohexadiene (161, n = 3) is formed in a related system, from the reaction of 1-bromocyclohexene with potassium t-butoxide in dimethyl sulphoxide, is shown by trapping it by the highly reactive 1,3-diphenylisobenzofuran (162). The Diels-Alder adduct (163), differs from that of cyclohexyne (164) which was obtained from 1,2-dibromo-cyclohexene and Mg in the presence of (162). The product (164) did not isomerize to (163) (Wittig and Fritze, 1966). 

Besides by these epoxidations, oxaspiropentanes have been prepared through the nucleophilic addition of 1-lithio- 1-bromocyclopropanes to ketones at low temperature. Thus for example, the dibromocyclopropane 96 prepared by addition of dibromo-carbene to cyclohexene 52) underwent metalation with butyllithium to give the lithio-bromocyclopropane 97 which was converted into the oxaspiropentane 98 upon simple addition to cyclohexanone, Eq. (28) 53,54). 

It is well known that in cyclohexan series (26, n = 6) this elimination is very difficult to perform with classical bases26). For example, to our knowledge, no satisfactory method has so far been found for preparation of 1-bromo cyclohexene from trans 1,2-dibromo cyclohexane. This apparently very simple reaction, leads only with great difficulties to the desired compound (yields are low and tedious purifications are required to obtain a pure product27)). 

A simple and efficient method for the debromination of vzc-dibromidcs to ( )-alkenes utilized the Sm-TMSC1-H20 catalytic system (Scheme 8.19). For example, franj-stilbene was produced from 1,2-dibromo-l,2-diphenylethane within 5 h in good yield at room temperature. The benzylic vz odibromides similarly gave the corresponding ( )-alkenes in a high yield. In the case of 1,2-dibromocyclohexane, a longer reaction time was needed to obtain cyclohexene. This is perhaps because the radical or anion intermediate of an aliphatic... 

For example, the reaction of methyllithium (from bromomethane) with dibromomethane and cyclohexene gave 7-bromobicyclo[4.1.0]heptane in 0.35-1% yield only. Alternatively, the reaction of bromoform with methyllithium (from chloromethane) and cyclohexene gave a mixture of 7,7-dibromo- (7%) and 7-bromobicyclo[4.1.0]heptane (1%). There are many competitive reactions observed in the dihalomethane or haloform and alkyllithium systems deprotonation, halogen-metal exchange in substrates and in intermediates, alkylation etc. (for examples see refs 28 and 29). These processes are described in detail in Houben-Weyl, Vol. 4/3, pp 225-228 and Vol. E19b, pp 1601-1602. Therefore, the reaction of dibromomethane or bromoform with an alkyllithium and an alkene cannot be seriously considered as a viable preparative synthesis of bromocyclopropanes. 

The thermal decomposition of bis[dibromo(trimethylsilyl)methyl]mercury in the presence of diphenylmercury occurs more easily than for the chloro analogs. However, 1-bromo-l-trimethylsilylcyclopropanes 3, which are formed in the presence of cyclohexene, are thermally less stable and partially decompose under the reaction conditions. ... 

The photochemical addition of ethene at 0°C in methylene chloride to the enedione (77) affords a high yield of the adduct (78). This was converted to the monochloro derivative (79) which also undergoes photoaddition of ethene to yield the Z> adduct (80). This on elimination of HCl yielded the quinol (81) which can be oxidised to the quinone (82). Cycloaddition of alkenes (cyclopentene, cyclohexene, and cycloheptene) has been carried out to the same enedione (77) to yield the adducts (83). lyoda et al. have also described a convenient synthesis of the bicyclo-octanediones(84) by a photochemical addition of alkenes to the enedione (77). The adducts (84) can be reduced by zinc in acetic acid to the desired products. Cycloaddition of ethyne to the same enedione followed by reduction affords the bicyclooctanes (85). The photoaddition of alkenes to the dibromo-enedione (86) is also effective and yields, after reduction, the adducts (87). 

The first clear case of a cyclopropanation via a carbene was presented by Doering and Hoffmann in 1954. They added trichloro- or tribromomethane and a base to a nonaqueous solution of cyclohexene and obtained 7,7-dichloro- (or 7,7-dibromo)bi-cyclo[4.1.0]heptane (norcarane, 8-2). 

To a distillation flask were added 107 g l,8-dibromo-3,7-dimethyl-9-(2,6,6-trimethyl-l-cyclohexen-l-yl)-2,6-nonadien-4-yne, 100 mL toluene, and 83 g triethyl phosphite. The resulting mixture was heated at 145°C for 4 h. Upon removal of the solvent, the residue was distilled through a centrifugal molecular still to give 80.9 g 3,7-dimethyl-9-(2,5,6-trimethyl-l-cyclohexen-l-yl)-2,6,8-nonatrien-4-yn-l-ylphosphonic acid diethyl ester, in a yield of 70%, m.p. 28°C. 

Another product formed by the reaction of bromine with cyclohexene in acetic acid is tra s-l-acetoxy-2-bromocyclohexane. The yield of this product varies with the concentration of added LiBr (with iorric strength held constant). With [LiBr] = 0, Brown and co-workers found that the product mixture consisted of 27% 1,2-dibromo and 73% l-acetoxy-2-bromo adducts. At [LiBr] 0.1 M, 90% of the product was 1,2-dibromocyclohexane and only 10% was the l-acetoxy-2-bromo derivative. Based on the conclusion that the low a-sec-ondary deuterium KIE requires a nucleophilic (not electrophilic) role for Br, Brown and co-workers proposed the detailed mechanism shown in Figure 9.5. Here, CTC is a charge transfer complex IIP and IIP are intimate ion pairs SSIP and SSIP are solvent-separated ion pairs DI is a dissociated ion and SOH is a hydroxylic solvent. The key feature of the mechanism is the necessity for Br migration to occur in the rearrangement of IIP to IIP so that backside attack can produce the dibromo product. The SSIP can rearrange to SSIP, but the latter must then reorganize to form IIP (so that the Br is inside the solvent shell) before nucleophilic attack can occur. Added Br can react with the CTC or with IIP to produce the dibromo product. 

The mechanism in Figure 9.5 also provides a rationalization for some observations of solvent effects in bromine addition reactions. The major product of the reaction of bromine with cyclohexene in methanol is trans-2-bromo-l-methoxycyclohexane. traMS-l,2-Dibromocyclohexane is observed if Br is added to the solution, but the yield of the dibromo adduct approaches 0% as [Br ] approaches 0 M. This result stands in contrast to the 27% of 1,2-dibromo adduct obtained from the corresponding reaction in acetic acid. It appears that the greater polarity of methanol accelerates the dissociation of IIP to DI, and the greater nucleophiliciW of methanol enhances the reaction of solvent with IIP, SSIP, DI, and SSIP. ... 

The reaction of bromine with cyclohexene involves anti addition, which generates, initially, the diaxial conformation of the addition product that then undergoes a ring flip to the diequatorial conformation of rn r-l,2-dibromo-cyclohexane. However, when the unsaturated bicyclic compound I is the alkene, instead of cyclohexene, the addition product is exclusively in a stable diaxial conformation. Account for this. (You may find it helpful to build handheld molecular models.)... 

The cyclohexene (58 X = H), after bromination, has been hydroxylated via adsorption on silica gel and contact with ozone at 195 K. Debromination of the resulting dibromo-alcohol with zinc in acetic acid gives a 30 % overall yield of the alcohol (58 X = OH). The stereoselectivity in cyanohydrin formation has been... 

In the previous section, the C=C unit of an alkene reacted as a Br0nsted-Lowry base with a Br0nsted-Lowry acid. These are, of course, acids that have a proton (see Chapter 6, Sections 6.2.1 and 6.2.4). Can the alkene react as a Lewis base The answer is yes (see Chapter 6, Section 6.4.5). With this in mind, consider the following experiment. When cyclohexene is mixed with elemental bromine in carbon tetrachloride as a solvent, the product is frans-l,2-dibromo-cyclohexane (38) isolated in 57% yield. In this case, quite a bit of unreacted cyclohexene is recovered. 

---